1. Field of the Invention
The present invention relates to an image forming apparatus that forms an electrostatic image on an image by means of, for example, an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
As a device for detecting a remaining amount of developer in an image forming apparatus, there has been one that is shown in FIG. 15, for example. More specifically explaining this device, a magnetic single component developer in a developing container 70 is sent to a developing chamber 73 by means of a developer feeding member 72. The developing chamber 73 has a sleeve 75 which internally contains a fixed magnet 74 therein and which rotates in a direction of an illustrated arrow, the sleeve 75 being arranged in opposition to a photosensitive drum 76. Also, on the sleeve 75, there is arranged an elastic blade 77 for coating the developer sent into the developing chamber 73. The distance between the sleeve 75 and the photosensitive drum 76 is in the range of 50μ-500μ, and a developing bias, which is generated by superposing an alternating current on a direct current by means of a developing bias power supply 101, is impressed on the sleeve 75, thereby performing so-called jumping development.
Next, reference will be made to a method for detecting the remaining amount of developer in the developing container 70 as explained above. A reference numeral 78 denotes an antenna composed of a rod which is made of metal such as stainless steel, etc., and which is arranged in parallel to the sleeve 75. When a developing bias is impressed on the sleeve 75, a voltage is induced in the antenna 78 under the action of an electrostatic capacitance between the sleeve 75 and the antenna 78. Here, note that the voltage induced in the antenna 78 depends on the electrostatic capacitance between the sleeve 75 and the antenna 78. Accordingly, the electrostatic capacitance between the sleeve 75 and the antenna 78 varies between a state where the amount of toner is sufficient to fill a space between the antenna 78 and the sleeve 75 with the developer, and a state where the toner has been consumed with the space between the sleeve 75 and the antenna 78 being not filled with the developer. Therefore, the voltage induced in the antenna 78 varies.
Generally, in a developing unit using a non-magnetic single component developer, a developer holding member is arranged in the development chamber 73. In a case where a developer remaining amount detection method using a change in an electrostatic capacitance is applied to a developing unit using such a non-magnetic single component developer, there will arise problems such as a narrow space for arranging an antenna, a hindrance to the conveyance of the developer, and so on, due to the provision of a coating member.
In order to solve the problems, it is known to use a roller-shaped member that supplies a developer to a sleeve which acts as a developer carrying member, as shown in FIG. 16 (e.g., Japanese Patent Application Laid-Open No. H04-234777). A feed member 80 is constructed to have a urethane sponge arranged on the circumference of a metal support member 79 having electrical conductivity. In addition, when the developer is coated onto a sleeve 95 by means of the feed member 80, a voltage corresponding to an amount of the developer is generated on the electrically conductive support member 79 by impressing an alternating voltage on the sleeve 75. A remaining amount of the developer is detected by the voltage thus induced.
In the method for detecting the voltage induced between the feed member 80 and the sleeve 75 by impressing an alternating voltage therebetween, as shown in the first patent document, a correlation is utilized between the developer contained in the feed member 80 and the amount of developer in the developing container. It is possible to detect the amount of remaining developer in the developing container from such a correlation.
However, it has been found that to perform successive detection of the amount of the remaining developer in the above-mentioned developing unit might sometimes be difficult. This is because the amount of toner in the above-mentioned feed member is not stable if the speed of image formation varies.
In the following, this will be described in detail. In general, in an image forming apparatus using a non-magnetic single component developing method, in a case where a recording medium is cardboard (i.e., paper for high quality pictures generally having a weight of 100 g/m2 or more) or the like, the apparatus performs an operation such that the speed of the recording medium passing a corresponding fixing unit is controlled to be slowed down for improved fixing performance. At that time, a development operation might be performed at a leading end portion of the recording medium at the time when the recording medium leading end portion begins to enter the fixing unit. In such a case, it is general to provide a plurality of so-called low-speed print modes in addition to a normal-speed print mode that corresponds to the image formation of plain paper (i.e., paper generally having a weight of about 60-80 g/m2). In the low-speed print modes, the rotational speeds of the photosensitive drum, the developing roller, and the above-mentioned developer feed member (hereinafter, feed roller) are dropped, too, following the decreased speed of the recording medium. Hereinafter, in the description that the rotational speed of the feed roller has been changed, it is assumed that the rotational speed of the developing roller is also changed while keeping a constant circumferential speed ratio with respect to the feed roller. When the rotational speed of the feed roller is changed in this manner, an amount of the developer held by the feed roller and the time required until a predetermined amount of developer is held thereby are changed in accordance with the rotational speed of the feed roller.
For example, as the rotational speed of the feed roller slows relatively, the amount of the developer stored in the feed roller increases with respect to the amount of the developer in the developing container. This is because when the feed roller takes in and releases the developer in a portion thereof abutting the developing roller, a force to release the developer becomes relatively weaker than a force to take in the developer as the rotational speed of the feed roller decreases.
In addition, on the other hand, as the rotational speed of the feed roller slows, the time required when the feed roller takes in and accumulates a predetermined amount of developer, which is decided by the amount of developer in the developing container and the rotational speed of the feed roller, becomes relatively longer. This is because the rotational speed of the feed roller slows, and the frequency per unit time and the ability of the developer intake and release operations in the abutting portions of the feed roller and the developing roller decrease.
Therefore, the time required until the feed roller accumulates the developer in a stable manner at the time when the rotational speed of the feed roller changes is not uniquely decided simply by the distance the feed roller and the developing roller have moved with respect to each other.
On the contrary, when the rotational speed of the feed roller becomes relatively faster, the amount of the developer in the feed roller decreases quickly up to a predetermined amount of developer, corresponding to the amount of the developer in the developing container and the rotational speed of the feed roller.
Due to the above-mentioned phenomenon, the variation of the electrostatic capacitance becomes gentle or gradual when the speed of image formation or the rotational speed of the feed roller slows relatively as in the low-speed print modes. That is, even if the amount of the developer in the developing container is constant, the electrostatic capacitance between the feed roller and the developing roller comes to vary in a gentle or gradual manner in accordance with the rotational speed of the feed roller. Therefore, when the developer is consumed while the image formation speed is frequently changed, the above-mentioned electrostatic capacitance does not take a fixed value corresponding to the amount of the developer in the developing container. As a result, it becomes difficult to detect an amount of change in the electrostatic capacitance corresponding to the amount of remaining developer, and hence it becomes difficult to perform successive detection of the amount of remaining developer.
For this phenomenon, it is considered to take a countermeasure of providing remaining amount detection tables for the individual rotational speeds of the feed roller, respectively, for example. However, when the rotational speed of the feed roller becomes slow, it is difficult to cope with the phenomenon by taking such a countermeasure. The method might still be effective if the above-mentioned electrostatic capacitance quickly stabilizes to an electrostatic capacitance inherent to the rotational speed of the feed roller when the rotational speed becomes slow. However, in actuality, when the rotational speed of the feed roller has been changed to a low-speed side, the developer is being filled into the feed roller little by little, so the electrostatic capacitance rises in a gradual manner. Accordingly, when the rotational speed of the feed roller has been changed into a low speed, it takes a long period of time until the electrostatic capacitance becomes a fixed value, so the output value does not become stable in a short period of time. In a case where during that time, a lot of developer has been consumed or the speed change has been frequently repeated, no stable electrostatic capacitance detection output value at each speed corresponding to the consumption of the developer is obtained, and successive remaining amount detection becomes difficult.